Install tox in a virtualenv

Since TravisCI has an extremely old version of PyPy, we need to use
pyenv to install a more recent version. However, pyenv's shims don't
persist between the install and script portions of Travis. So we have to
use a virtualenv so Travis can find the tox command.

Author: sigmavirus24

.travis.yml

@@ -25,4 +25,5 @@ install:
     - ./.travis/install.sh
 
 script:
+  - source ~/.venv/bin/activate
   - tox

.travis/install.sh

@@ -14,4 +14,6 @@ if [[ "${TOXENV}" = pypy* ]]; then
 fi
 
 
+python -m virtualenv ~/.venv
+source ~/.venv/bin/activate
 pip install --upgrade tox